Many appliances, such as furnaces, use pilot lights for igniting the main burner of the appliance. For example, in a high efficiency furnace, a pilot light or igniting flame is ignited by a spark or electrically heated ignitor in response to a request for heat signal from a thermostat. This igniting flame provides the energy to ignite the fuel (e.g., natural gas) and air mixture at the combustion chamber of the furnace. However, it is important that the igniting flame is present before the fuel valve of the furnace is opened to provide fuel to the combustion chamber. Thus, the control system for the fuel valve must include a system for ensuring that an igniting flame is present when required to ignite the fuel-air mixture at the combustion chamber.
One way to sense the presence of a flame is to provide a voltage potential between two electrodes (e.g., flame hood and electrode near the tip of the flame), both located within a flame area (the area occupied by the ionized gases of a flame when a flame is present). The current flow within the flame area between the electrodes is monitored and will exceed a certain threshold when a flame is present due to the conductivity of the ionized gases in the flame area, By way of example, a typical furnace would apply 24 volts to the electrodes and a current of 50 or more nanoamps would indicate that a flame is present.
Electronics for accurately sensing currents in the range of 50 nanoamps can be relatively sensitive, since noise can substantially influence such sensing. Furthermore, circuits for flame current sensing in furnaces must be fail-safe for safety reasons. Accordingly, to provide reasonably priced fail-safe circuits for sensing flame current, circuits have been produced which only give a binary signal (flame present) based upon the presence or absence of a threshold flame current.
Flame current sensing circuits which only indicate that a flame is present or absent fulfill the primary need of flame detection; however, these circuits do not provide any information about the value of the flame current other than that it is above or below a setpoint.
For purposes of maintaining the electrodes of a flame current sensing circuit, and troubleshooting, it would be useful to have more information about the value of the flame current. For example, a typical problem with flame current sensing circuits is that the electrodes form a resistive layer over time due to oxidation and carbon deposits. When the resistance caused by such deposits becomes too great, the flame current is reduced and the circuit determines that a flame is not present, regardless of the presence of a flame, and prevents the furnace from operating. One solution to this problem is to clean the electrodes. However, this may only solve the problem temporarily if one or both of the electrodes were not sufficiently cleaned. Thus, it would be desirable to know how much the flame current exceeds the setpoint for purposes of checking electrode performance and predicting electrode cleaning schedules.
Accordingly, it would be useful to provide a simple, low-cost flame sensing circuit which could produce output signals representative of more than one flame current level and, preferably, output signals representative of a range of flame current levels.